This Resource supports a number of projects using gas chromatography/mass spectrometry and associated computer methods. Selected ion monitoring under computer control is used extensively as a method for quantitative analysis of biological compounds at high precision or very low levels, down to a few picograms in several instances. Myoinositol is measured in single nerve cells or neuroanatomical regions of similar size. A gradient of distribution in the cochlear nucleus has been established. Regional levels of morphine in neural tissue are measured to correlate with effective analgesia. An assay of catecholamines in plasma is being developed. Stable isotope tracers are used to study fuel fluxes and the etiology of hypoglycemia in children, and the effects on glucose homeostasis from muscle wasting in patients with muscular dystrophy. Inborn errors of metabolism are detected by identifying abnormal metabolites in urine. Abnormalities in the fatty acid composition of cell membranes are also under study. The pharmacokinetics of anticonvulsant drugs administered to newborns and children are measured to determine effective dose levels and clearance times. Peptide sequencing by chemical ionization is being investigated using mixtures of dipeptides obtained by hydrolysis with diaminopeptidases. A negative ion source and detection system is being designed. Plasma levels of tricyclic antidepressant drugs are measured in a clinical population. These levels are correlated with therapeutic effectiveness and reactions to overdose.